Task 1: Detect and Report Corruptions

The first task is the detection and reporting of corruptions. Reporting not only indicates what is wrong with a block, but also identifies the associated repair directive. There are several ways to detect corruptions. Table 25-1 describes the different detection methodologies.

Table 25-1 Comparison of Corruption Detection Methods

Detection Method

Description

DBMS_REPAIR PL/SQL package

Performs block checking for a specified table, partition, or index. It populates a repair table with results.

DB_VERIFY utility

Performs block checking on an offline database

ANALYZE TABLE SQL statement

Used with the VALIDATE STRUCTURE option, the ANALYZETABLE statement verifies the integrity of the structure of an index, table, or cluster; checks or verifies that tables and indexes are synchronized.

DB_BLOCK_CHECKING initialization parameter

When DB_BLOCK_CHECKING=TRUE, corrupt blocks are identified before they are marked corrupt. Checks are performed when changes are made to a block.

DBMS_REPAIR: Using the CHECK_OBJECT and ADMIN_TABLES Procedures

The CHECK_OBJECT procedure checks and reports block corruptions for a specified object. Similar to the ANALYZE...VALIDATE STRUCTURE statement for indexes and tables, block checking is performed for index and data blocks.

Not only does CHECK_OBJECT report corruptions, but it also identifies any fixes that would occur if FIX_CORRUPT_BLOCKS is subsequently run on the object. This information is made available by populating a repair table, which must first be created by the ADMIN_TABLES procedure.

After you run the CHECK_OBJECT procedure, a simple query on the repair table shows the corruptions and repair directives for the object. With this information, you can assess how best to address the reported problems.

DB_VERIFY: Performing an Offline Database Check

Use DB_VERIFY as an offline diagnostic utility when you encounter data corruption.

ANALYZE: Reporting Corruption

The ANALYZE TABLE...VALIDATE STRUCTURE statement validates the structure of the analyzed object. If the database encounters corruption in the structure of the object, then an error message is returned. In this case, drop and re-create the object.

You can use the CASCADE clause of the ANALYZE TABLE statement to check the structure of the table and all of its indexes in one operation. Because this operation can consume significant resources, there is a FAST option that performs a lightweight check. See "Validating Tables, Indexes, Clusters, and Materialized Views" for details.

DB_BLOCK_CHECKING Initialization Parameter

You can enable database block checking by setting the DB_BLOCK_CHECKING initialization parameter to TRUE. This checks data and index blocks for internal consistency whenever they are modified. DB_BLOCK_CHECKING is a dynamic parameter, modifiable by the ALTER SYSTEM SET statement. Block checking is always enabled for the system tablespace.

Task 2: Evaluate the Costs and Benefits of Using DBMS_REPAIR

Before using DBMS_REPAIR you must weigh the benefits of its use in relation to the liabilities. You should also examine other options available for addressing corrupt objects. Begin by answering the following questions:

What is the extent of the corruption?

To determine if there are corruptions and repair actions, execute the CHECK_OBJECT procedure and query the repair table.

What other options are available for addressing block corruptions? Consider the following:

If the data is available from another source, then drop, re-create, and repopulate the object.

Issue the CREATE TABLE...AS SELECT statement from the corrupt table to create a new one.

Ignore the corruption by excluding corrupt rows from SELECT statements.

Perform media recovery.

What logical corruptions or side effects are introduced when you use DBMS_REPAIR to make an object usable? Can these be addressed? What is the effort required to do so?

It is possible that you do not have access to rows in blocks marked corrupt. However, a block can be marked corrupt even if there are rows that you can validly access.

It is also possible that referential integrity constraints are broken when blocks are marked corrupt. If this occurs, then disable and reenable the constraint; any inconsistencies are reported. After fixing all problems, you should be able to reenable the constraint.

Logical corruption can occur when there are triggers defined on the table. For example, if rows are reinserted, should insert triggers be fired or not? You can address these issues only if you understand triggers and their use in your installation.

If indexes and tables are not synchronized, then execute the DUMP_ORPHAN_KEYS procedure to obtain information from the keys that might be useful in rebuilding corrupted data. Then issue the ALTER INDEX...REBUILD ONLINE statement to synchronize the table with its indexes.

If repair involves loss of data, can this data be retrieved?

You can retrieve data from the index when a data block is marked corrupt. The DUMP_ORPHAN_KEYS procedure can help you retrieve this information.

Task 3: Make Objects Usable

DBMS_REPAIR makes the object usable by ignoring corruptions during table and index scans.

Corruption Repair: Using the FIX_CORRUPT_BLOCKS and SKIP_CORRUPT_BLOCKS Procedures

You can make a corrupt object usable by establishing an environment that skips corruptions that remain outside the scope of DBMS_REPAIR capabilities.

If corruptions involve a loss of data, such as a bad row in a data block, all such blocks are marked corrupt by the FIX_CORRUPT_BLOCKS procedure. Then you can run the SKIP_CORRUPT_BLOCKS procedure, which skips blocks that are marked as corrupt. When the SKIP_FLAG parameter in the procedure is set, table and index scans skip all blocks marked corrupt. This applies to both media and software corrupt blocks.

Implications when Skipping Corrupt Blocks

If an index and table are not synchronized, then a SET TRANSACTION READ ONLY transaction can be inconsistent in situations where one query probes only the index, and a subsequent query probes both the index and the table. If the table block is marked corrupt, then the two queries return different results, thereby breaking the rules of a read-only transaction. One way to approach this is not to skip corruptions in a SET TRANSACTION READ ONLY transaction.

A similar issue occurs when selecting rows that are chained. A query of the same row may or may not access the corruption, producing different results.

Task 4: Repair Corruptions and Rebuild Lost Data

After making an object usable, perform the following repair activities.

Recover Data Using the DUMP_ORPHAN_KEYS Procedures

The DUMP_ORPHAN_KEYS procedure reports on index entries that point to rows in corrupt data blocks. All such index entries are inserted into an orphan key table that stores the key and rowid of the corruption.

After the index entry information has been retrieved, you can rebuild the index using the ALTER INDEX...REBUILD ONLINE statement.

Fix Segment Bitmaps Using the SEGMENT_FIX_STATUS Procedure

Use this procedure if free space in segments is being managed by using bitmaps (SEGMENT SPACE MANAGEMENT AUTO).

This procedure recalculates the state of a bitmap entry based on the current contents of the corresponding block. Alternatively, you can specify that a bitmap entry be set to a specific value. Usually the state is recalculated correctly and there is no need to force a setting.